1. Field of the Invention
The present invention is directed to probe-based instruments and, more particularly, relates to a scanning probe microscope (SPM) based method and apparatus for facilitating high speed measurements of a characteristic of a sample feature, such as the maximum width of a semiconductor via, a minimum width of a semiconductor line, or the roughness of a semiconductor line edge.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs) typically characterize the surface of a sample down to atomic dimensions by monitoring the interaction between the sample and a tip on the cantilever probe. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and which has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography, elasticity, or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Alternatively, some AFMs can at least selectively operate in an oscillation mode of operation such as TappingMode™ (TappingMode is a trademark of Veeco Instruments, Inc.) operation. In TappingMode™ operation the tip is oscillated at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample. The probe is moved at a fixed speed over the sample to collect a set number, e.g., 512, of data points per line, hence dividing the line into 511 equally spaced regions. Each data point represents an average height in that region.
Another form of oscillating mode operation, known as Critical Dimension mode or simple “CD” mode, distributes data acquisition along a scan profile so as to maximize data acquisition points in areas of changing topography. The scan rate is not constant as in TappingMode because the scanner instead adapts itself to the topography of the sample surface. The regions between the data points therefore are not equally spaced but, instead, are more heavily distributed over areas of the changing topography. For example, of 500 data points taken along a scan profile, 300 data points may be taken over one-fourth of the total length of that line. As a result, the actual height is determined in critical regions of the sample as opposed to simply determining an average height over the entire region.
AFMs and other SPMs are being used with increasing frequency for measuring characteristics of features of semiconductor devices and other devices with high accuracy and repeatability. For example, several operational characteristics of a semiconductor wafer are dependent upon the extreme dimensions of features formed on or in the wafer's surface. Semiconductor device manufacturers demand that these characteristics be known with a high degree of precision. These characteristics include, but are not limited to “extreme dimensions” such as the maximum widths of vias and trenches and the minimum widths of lines. A line is a semiconductor structure expanding upwardly substantially perpendicular to the top surface of the wafer. A trench is an elongated depression etched into or otherwise formed in a dielectric surface of a wafer. Trenches are often filled with trench capacitors. A “via” is a hole which is etched or otherwise formed in the interlayer dielectric of a wafer. It may be filled with a conducting material, such as metal, to allow for the electrical connection of several layers on the semiconductor, typically either tungston or copper.
Via metrology in semiconductor processing is important for the development and production of fast, efficient integrated circuits (ICs). For instance, the critical dimensions of vias are intimately related to chip speed. Chip speed is proportional to the rate at which switches are able to change between “on” and “off” states. This switching speed is inversely proportional to it's circuit's time constant, RC where C is the capacitance of the semiconductor device that is switching states and R is the resistance of the via that is permitting the dissipation of charge stored in that capacitor and is inversely proportional to the vias cross sectional area. This resistance is also intimately tied to the continuity of the conductive material that will fill the via. When the sidewalls of the via are sloped, voids and defects are more likely to occur during the fill process, causing higher resistance along the via, if not breaking conductivity all together, thus reducing device/chip speed. Thus, in order to develop fast, efficient ICs, knowledge of top, middle and bottom via widths is helpful for optimizing device, circuit and chip performance.
A semiconductor via V is shown somewhat schematically in greater detail in FIG. 1A-1C. It is generally in the shape of a truncated hemisphere, having a maximum depth Dmax near its center. At any given location along the depth of the via V, it will also have a maximum width Wmax. Three maximum widths Wmaxtop, Wmaxmiddle, and Wmaxbottom are illustrated in FIGS. 1B and 1C by way of example in the upper, central, and lower regions of the via. Techniques have been proposed to measure extreme dimensions of semiconductor characteristics such as a via's maximum width using an AFM. However, these techniques have proven less than optimal.
For instance, in one approach utilized by the SXM software incorporated into some AFMs available from Veeco Instruments Inc., a high-resolution scan is taken over an area of the semiconductor surface containing the feature of interest. As is typical with such scans, data is obtained by moving the probe back and forth relative to the sample in a primary or X direction while incrementing the probe relative to the sample in the Y direction between passes in the X direction. The resultant scan secures data along profiles P or lines “L” in the X direction, with the adjacent lines being separated by a gap ΔYINC in the Y direction as is seen in FIG. 2A. The length of each scan line, spacing between adjacent scan lines, and number of scan lines are user-defined. The length and height of the scan may range from considerably less than 1 micron to 4 microns or more. The resolution of the data is dependent in part upon the length of the increments ΔYINC or stated another way, of the density of the scan in the Y direction. High-resolution scans in CD mode typically involve 35-60 scan lines per micron, typically resulting in the acquisition of 16-32 scan lines passing through the feature of interest. After taking such a high-resolution scan through the feature of interest, the software simply selects the scan line having the greatest or smallest length in the X direction as the maximum or minimum length of the feature of interest. Once such line is designation LMAX APP in FIG. 2A.
The technique described above is relatively time consuming because it requires a high-resolution scan. It also leads to relatively rapid tip wear—an important consideration where AFM tips are employed that may cost $1,000 or more. Perhaps somewhat counterintuitively, the repeatability is also relatively poor even at high-resolution. The reasons for this characteristic can be appreciated with reference to FIG. 2B. That figure shows a portion of a sidewall edge of the via V of FIG. 2A that includes the point PACT at which the line LMAX ACT of greatest maximum length actually passes. The line LMAX ACT lies between the line LMAX APP that is identified by software as the line of maximum length and the next adjacent line, resulting in an offset ΔY between LMAX ACT and LMAX APP. The likely magnitude of the offset ΔY, as reflected by reduced repeatability, is inversely related to the resolution of the scan. Hence, while some users may adopt a scan density of as little as 4 scan lines passing through the feature of interest to maximize throughput, the repeatability of the resulting measurement is extremely poor. In addition, noise in the measurement resulting from scanner hysteresis and other factors may result in the acquisition of data on an apparent surface SAPP that deviates from the actual surface SACT, resulting in the determination of an apparent end point PAPP of the line LMAX APP that is offset from the actual point PACT on the actual surface SAPP by a factor ΔX. The offset ΔY could be reduced by increasing resolution still further (at the cost of reduced speed and increased tip wear), but the offset ΔX could be the same or even worse. Indeed, the increased measurement period required for high-resolution scans reduces accuracy because temperature changes and other environmental variations that occur over time can lead to increased measurement variations.
Another technique, proposed by IBM, attempts to determine the maximum width of a via or similar sample feature by obtaining a high-resolution scan as described above and then interpolating the measured width between scan lines to obtain the location of the actual longest scan line as opposed to simply selecting a scanned line. However, this technique, like the technique described above, is time consuming and subject to rapid tip wear. It interpolates width, so it assumes that width variation is uniform or can be modeled by a fixed polynomial order. It also assumes that both edges of the via are of symmetrically identical shape—an assumption that often is inaccurate. The interpolation procedure utilized by this technique also varies with the scans and, accordingly, does not have good repeatability.
In light of the foregoing, the need exists to measure a characteristic of a feature of a sample, such as a maximum or minimum width, rapidly and with high levels of repeatability and accuracy.